Recently, a semiconductor memory such as a magnetic random access memory (MRAM) using a variable resistance element as a memory element is attracting attention and being developed. The MRAM uses, as a memory element, a magnetic tunnel junction (MTJ) element using the magnetoresistive effect by which the resistance value changes in accordance with the magnetization direction. In particular, a large resistance change is obtained by a tunneling magnetoresistive (TMR) element using the TMR effect.
The TMR element has a structure in which two ferromagnetic layers sandwich a nonmagnetic layer (insulating layer). While the magnetization direction in one ferromagnetic layer (pinned layer) is fixed, the magnetization direction in the other ferromagnetic layer (free layer) is not fixed; the magnetic direction in the free layer is parallel or antiparallel to that in the pinned layer. The TMR effect is a phenomenon in which the resistance of the TMR element changes depending on the relative relationship (parallel/antiparallel) between the two magnetization directions. More specifically, a current readily flows through the insulating layer (the resistance decreases) when the magnetization direction in the free layer is parallel to that in the pinned layer, and hardly flows (the resistance increases) when the former is antiparallel to the latter. A memory element from which data written in the TMR element can be read in accordance with the resistance can be formed by making the relative relationship between the two magnetization directions correspond to “0” or “1”.
The TMR element has a resistance value Rmin or Rmax (Rmax>Rmin) in accordance with whether the magnetization directions in the free layer and pinned layer are parallel or antiparallel. In a read operation of the MRAM, it is necessary to supply a read current or apply a read voltage to a memory cell as a read target, and read data by comparing the change in voltage or current corresponding to the resistance value of the TMR element with a reference signal. The reference signal is formed from an external circuit or from a reference cell in which data “0” or “1” is prewritten. However, the method of forming the reference signal from an external circuit has the problem that the method requires extra space and extra power consumption, and it is necessary to reproduce characteristics that follow the temperature characteristics of the TMR element.
Accordingly, even when reading data from the MRAM by using the reference signal, it is desirable to generate the reference signal by using the TMR element. As a method of generating this reference signal, a method using middle resistance Rmid=(Rmax+Rmin)/2 of the TMR element has been disclosed (Jpn. PCT National Publication No. 2005-501370). In this method, however, the reference current is not middle between a current flowing through the resistance value Rmin and a current flowing through the resistance value Rmax, and as a consequence the sense margin decreases.
Also, in the MRAM using the spin transfer method, as a current is supplied to the TMR element in a read operation in the same manner as in a write operation, a so-called read disturbance by which a write error occurs during data read is highly likely to occur. Especially in the method of generating the reference current from a reference cell in a read operation, the reference cell is accessed more frequently than a cell as a read target. This increases the probability that the reference cell suffers the read disturbance.